1. Field of the Invention
The present invention relates to a semiconductor device provided with a dynamic random access memory (DRAM) cell having a trench capacitor and a method of manufacturing the same.
2. Description of the Related Art
With recent refinement of semiconductor devices, a higher and higher aspect ratio processing technique has been necessitated. Particularly in the manufacture of DRAM cells with a trench capacitor structure, the forming of a deep trench necessitates a higher aspect ratio processing technique. JP-A-2001-144265, JP-A-2001-217404 and JP-A-2002-222870 each disclose a technique for forming a deep trench into a bottle shape. An inside area of the trench can be increased when a deep trench is formed into a bottle shape. A capacity of the trench capacitor can be increased when a capacitor is then formed on an inner surface of the bottle-shaped trench.
Thus, since the trench is formed into the bottle shape, an upper part of the trench has a tapered section, that is, the upper part of the trench has a larger diameter than a lower part of the trench. However, an aspect ratio of the trench is increased with reductions in design rules and refinement of elements. As a result, it is difficult to control a taper angle of the upper part section of the trench. Accordingly, the taper angle is increased nearly to a vertical angle. For example, the taper angle sometimes exceeds 89° or 90°. When the taper angle exceeds 90°, the upper part section of the trench has the shape of an inverted taper or an overhung shape. The taper angle thus increased nearly to 90° results in a void formed in a conductive film buried deep in the trench and/or a seam formed in an upper part of the conductive film.
A collar insulating film needs to be formed on the conductive film and sidewall faces of the trench after the aforementioned trench forming step. The collar insulating film has a large thickness in order that electric charge may be prevented from being discharged from the conductive film. The collar insulating film is generally formed in the following manner. The conductive film is formed deep in the trench and thereafter, the collar insulating film is isotropically formed on the sidewalls of the trench and on the conductive film. The collar insulating film formed on the conductive film is then removed by an anisotropic etching process while the collar insulating film remains on the sidewalls of the trench. Thus, the collar insulating film is formed on the sidewalls of the trench.
As the taper angle of the trench upper section is increased nearly to a vertical angle, a void occurs in the conductive film and/or a seam is produced in the upper part of the conductive film. As a result, the collar insulating film remains on the conductive film even if the anisotropic etching is carried out to remove the collar insulating film isotropically formed on the conductive film. Subsequent to the anisotropic etching process, another conductive film needs to be further deposited on the conductive film in the trench so that the conductive film is electrically conductive to a transistor. However, when the collar insulating film remains on the conductive film formed deep in the trench, for example, the electric contact becomes insufficient even if the another conductive film is formed on the conductive film. This results in an increase in the resistance of an electrode or an electrically insulated state, whereupon a malfunction tends to occur in the semiconductor device.